disk: Check if returned pointer for allocated memory is NULL

disk: Check if returned pointer for allocated memory is NULL

When using grub_malloc(), grub_zalloc() or grub_calloc(), these functions can
fail if we are out of memory. After allocating memory we should check if these
functions returned NULL and handle this error if they did.

On the occasion make a NULL check in ATA code more obvious.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>